Active Current Surge Limiters

ABSTRACT

Active current surge limiters and methods of use are disclosed. One exemplary system, among others, comprises a current limiter, including an interface configured to be connected between a power supply and a load; a disturbance sensor, configured to monitor the power supply for a disturbance during operation of the load; and an activator, configured to receive a control signal from the disturbance sensor and to activate the current limiter based on the control signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to co-pending U.S. provisionalapplication entitled, “System and Method for Determining Power SystemTransmission Line Information,” having Ser. No. 60/648,466, filed Jan.31, 2005, which is entirely incorporated herein by reference.

TECHNICAL FIELD

The present disclosure is generally related to limiting current surgeand, more particularly, embodiments of the present disclosure arerelated to actively limiting surge current produced by power supplydisturbances during load operation.

BACKGROUND

There are many applications where it is necessary to protect electricalequipment from power surges and high energy transients that could damageor adversely affect the operation of such equipment. Voltage surges arecommonly perceived to be the most common cause for damage to electricalequipment during operation. Voltage surges, such as those produced bylightning strikes, can cause large currents to flow resulting in damageto operating equipment. Electrical equipment utilizing electronics, suchas a rectifier front end, are particularly susceptible to damage. As aresult, transient voltage surge suppressors (TVSS) are commonly utilizedto clamp the voltage level and absorb energy associated with atransient. However, analysis strongly suggests that there is a fairlyhigh probability that equipment will be also be damaged by currentsurges that occur at the end of voltage sags. Furthermore, industrialstudies have indicated that voltage sags are much more likely to occurthan voltage surges. While TVSS devices limit the voltage applied toequipment, they do not limit the current surge experienced by electricalequipment at the end of voltage sag transients.

High inrush currents are also commonly experienced during the startingof electrical equipment. Inrush current limiting circuits, including anegative temperature coefficient (NTC) thermistor or resistor connectedbetween a power supply and a protected load and a bypass switch inparallel with the NTC thermistor, are often used to mitigate the currentsurge seen by the load during starting. A NTC thermistor is a componentwith a resistance that decreases as its temperature increases. Duringstartup, the temperature of the NTC thermistor is cold and itsresistance is high. As operation continues, the temperature increasesand the resistance of the NTC thermistor decreases, allowing morecurrent during normal operation. Once the equipment has completed itsstartup or a preset time has elapsed, the bypass switch closes to removethe resistor from between the power supply and the electrical load. Thecurrent limiter circuit remains disabled until the equipment isde-energized and the bypass switch is reopened. While the inrush currentlimiter circuits limit the current surge during startup, these inrushcurrent limiter circuits do not provide protection from electricaltransients during normal operation of the electrical equipment.

SUMMARY

Briefly described, embodiments of this disclosure, among others, includeactive current surge limiters and methods of use. One exemplary system,among others, comprises a current limiter, including an interfaceconfigured to be connected between a power supply and a load; adisturbance sensor, configured to monitor the power supply for adisturbance during operation of the load; and an activator, configuredto receive a control signal from the disturbance sensor and to activatethe current limiter based on the control signal.

Another exemplary system, among others, comprises means for limitingcurrent supplied to a load from a power supply; means for sensing adisturbance on the power supply during operation of the load; and meansfor activating the means for limiting current to the load when adisturbance is sensed.

Methods of use are also provided. One exemplary method, among others,comprises monitoring a condition of a power supply during operation of aload connected to the power supply; determining if the condition fallsoutside of an acceptable limit; and activating a current limiting devicewhen the monitored condition falls outside of acceptable limits.

Other structures, systems, methods, features, and advantages will be, orbecome, apparent to one with skill in the art upon examination of thefollowing drawings and detailed description. It is intended that allsuch additional structures, systems, methods, features, and advantagesbe included within this description, be within the scope of the presentdisclosure, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 illustrates an active current surge limiter.

FIG. 2 is an alternative embodiment of the active current surge limiterutilizing a microcontroller and semiconductor switches.

FIG. 3 is an alternative embodiment of the active current surge limiterutilizing a microcontroller and an electromechanical relay.

FIG. 4 is an alternative embodiment of the active current surge limiterutilizing a voltage detector and an electromechanical relay.

FIG. 5 is an alternative embodiment of the active current surge limiterutilizing an optocoupler and an electromechanical relay.

FIG. 6 is a flow chart illustrating an embodiment of a fast detectionalgorithm for the active current surge limiter.

DETAILED DESCRIPTION

Voltage sags have been shown to occur fairly frequently in industrialsettings. Studies indicate that voltage sags are 100 to 1000 times morelikely to occur than voltage surges. Data and analysis strongly suggesta high probability that operating equipment can be damaged by a currentsurge that occurs at the end of the voltage sag. The most vulnerablepoint for typical equipment is the end of short-duration sags, when theinrush limiting circuits are normally disabled. The current surge canhave excessively high I²T ratings because the normal inrush limitingcircuit (NTC thermistor or resistor+bypass switch) is disabled. Thecurrent surge causes damage to equipment, as well as degradation ofcomponents leading to shortened equipment life and premature equipmentfailure. Industrial, commercial and residential equipment that arepotentially subject to the problem include, but are not limited to,PC's, servers, TV's, stereo amplifiers, microwave ovens, PLC's, robots,machine drives, medical equipment, etc.

Embodiments of active current surge limiters are described below. Itshould be emphasized that the described embodiments are merely possibleexamples of implementations, and are set forth for clear understandingof the principles of the present disclosure, and in no way limit thescope of the disclosure.

FIG. 1 illustrates an active current surge limiter. The active currentsurge limiter 100 is connected at an interface between a power supply110 and a load 120. Power supplies include AC and/or DC sources. Whilethe principles discussed are generally applied to applications up to1000 Volts, this does not prevent their use in applications at highervoltage levels. Loads that are sensitive to these disturbances include,but are not limited to, industrial, commercial and residential equipmentthat include electronic components that operate with a DC power supply.A transient voltage surge suppressor (TVSS) 130 connected on the inputside can provide the added functionality of a voltage surge suppressordevice. The active current surge limiter 100 includes a current limiter140 for limiting the current supplied to the connected load 120, adisturbance sensor 150 for monitoring the condition of the power supply110, and an activator 160 for activating the current limiter 140 whenthe disturbance sensor detects a disturbance on the power supply.

Disturbances in the power supply can include variations in the powersupply characteristics such as, but are not limited to, the voltage,current, and combinations thereof. The presence of a power supplydisturbance is indicated when the sensed characteristic falls outsideestablished operational limits. Operational limits can be preset basedon variables such as, but not limited to, industrial standards and knownload and supply characteristics. However, as the power supply and loadcharacteristics are typically unknown, establishment of allowablecurrent limits can require additional analysis. Another alternative isto allow the disturbance sensor 150 to establish limits based oncontinuous monitoring of selected supply characteristics.

FIG. 2 is an alternative embodiment of the active current surge limiterutilizing a microcontroller and power semiconductor switches. Thisnon-limiting embodiment of an active current surge limiter 100, thedisturbance sensor 150 uses a microprocessor or microcontroller 200 toestablish allowable current limits, continuously monitor power supplycharacteristics (i.e. sensing voltage 205 and current 210), andcommunicate a control signal 215 to the activator 160 indicating thepresence of a disturbance on the power supply. The described controlstrategy allows the active current surge limiter 100 to handle power-upand load change without problems.

To establish the allowable current limit, the circuit in FIG. 2 sensesand measures the current 210 drawn by the load 120, including peakcurrent at start-up, through a current transformer 220. The peak currentat start-up is stored in a peak-rectifier circuit (not shown), includinga diode and capacitor coupled with a current transformer, and measuredby an A/D converter incorporated in the microcontroller 200. One skilledin the art would realize that other measurement circuits could also beutilized to measure power supply characteristics. The starting currentis recorded and stored by the microcontroller 200 as a peak inrushcurrent. During operation of the load 120, the microcontroller 200continues to monitor the load current 210 and record any sensed peakcurrents.

The microcontroller 200 also monitors the incoming ac line voltage 205.Limits for the sensed voltage 205 can be preset or established by themicrocontroller 200. Voltage sags occur when a supply voltage dropsbelow a predetermined level, such as but not limited to, 90% of ratedvoltage for short periods of time of one half cycle or more. When a sagin the monitored line voltage 205 is detected by the microcontroller200, a peak current limit reference (I_(max)) is set to the maximum peakcurrent value thus far recorded. During a voltage sag or momentaryinterruption, the current drawn by the load is most likely to decrease.At the end of the voltage sag, the voltage can quickly return to normal,causing a surge in the sensed current 210. The magnitude of the surgecurrent is affected by load factors, such as the type, condition, andproximity as well as power supply factors, such as magnitude andduration of disturbance, line impedance, return profile of the linevoltage, and transformer location. Industrial, commercial andresidential equipment vulnerable to the effects of current surgesinclude, but not limited to, PC's, servers, TV's, stereo amplifiers,microwave ovens, PLC's, robots, machine drives, and medical equipment.Moreover, any equipment utilizing rectifier/capacitor circuits amplifythe surge current effects when the capacitor is substantially dischargedduring a voltage sag.

Once the microcontroller 200 detects a current level that exceeds theI_(max) threshold, a control signal 215 is sent to the activator 160indicating the presence of a disturbance. In this non-limitingembodiment, the current limiter 140 is activated by turning off asemiconductor switch 225 through a gate drive 230. Activation of thecurrent limiter 140 forces the load current to flow through an acvoltage clamping device 235, such as but not limited to, a varistor. Thevoltage impressed across the load 120 is reduced, limiting the currentsupplied to the load. The switch 225 can then be turned on at, but notlimited to, the next cycle, a zero crossing point, and a predeterminednumber of switching under a high frequency duty cycle control scheme asis customary in PWM circuits. If the sensed current 210 remains high forgreater than a preset period of time, such as but not limited to one totwo seconds, then a trip signal 240 is activated by the microcontroller200, opening an overload switch or circuit breaker 245 and shutting thesystem down until a reset is effectuated, e.g., a reset button ispressed. Incorporation of a voltage clamping device 250 providesadditional voltage surge protection to the connected load 120.

The use of gate turn-off devices 225 allows turn-off and over-currentprotection even under normal voltage conditions as well as in thepresence of fast rising current fronts that occur under faultconditions. For successful operation, the components are sized to handletrapped energy in line and load inductances. In addition, powerdissipation during continuous operation should be considered duringselection.

FIG. 3 is an alternative embodiment of the active current surge limiterutilizing a microcontroller and an electromechanical relay. Thisnon-limiting embodiment utilizes the same disturbance sensor 150 tosense voltage 205 and current 210 as depicted in FIG. 2. During normaloperation, the current limiter 140 can be bypassed using anelectromechanical relay, contactor or switch. In this depiction, acontrol signal 215 sent by the microcontroller 200 causes a normallyopen relay 355 to close and deactivate the current limiter 140. Thepower supply is continuously monitored as described for FIG. 2.

Fast detection algorithms (e.g., as described in FIG. 6) allow thedetection of supply disturbances within one quarter to one half cycle.Fast detection algorithms can be implemented in, but not limited to,software, hardware and/or individual components. Because the linecurrent drawn by the load typically drops dramatically when the DCcapacitor reverse biases the diode bridge during a voltage sag, avoltage sag that is likely to cause inrush current can be can readilydetected. Upon detecting the onset of the voltage sag, the controlsignal 215 causes the relay 355 to open and activating the currentlimiter 140.

The current limiter 140 in this embodiment includes two resistors, 360and 365, with a thyristor pair or triac 370 connected in parallel withthe second resistor 365. Alternative combinations can also be utilized.Upon exceeding I_(max), resistors 360 and 365 provide a high resistanceto limit current to the attached load. After a sufficient time delay ora determination that the sensed current 210 is below an allowable level,the triac 370 is turned on, allowing higher current levels. Control ofthe triac 370 is provided by a signal 375 sent by the microcontroller200 to a gate driver 330 for the triac 370. Once the sensed current 210subsides or after sufficient time has elapsed, the relay 355 is reclosedallowing normal load operation to resume. As described for FIG. 2, ifthe sensed current 210 remains high for a predetermined period, a tripsignal 240 is activated by the microcontroller 200, opening an overloadswitch or circuit breaker 245 and shutting the system down.

With the use of a multi-step current limiter 140, it is possible tosignificantly improve the performance so as to minimize impact on theload. The level of surge current that flows in the system depends on anumber of parameters including, but not limited to, the depth andduration of the voltage sag, the load rating, the short circuit currentavailable at the load point, and the amount of capacitance in the loadrectifier. Monitoring of I_(max) provides an indication of the loadcharacteristics and maximum current necessary for normal operation. Thecurrent flowing through the resistors 360 and 365 forward biases thediode and provides an indication of the effective DC bus voltage(V_(dc)) in the load. If triac 370 is turned on at an angle α, thedifference between the line and DC bus voltages (V_(line)−V_(dc)) isapplied across resistor 360 and allowing an increase in current flow tothe load 120. Neglecting line and load inductances, the line currentdecreases until, at an angle β, it reaches to zero when the line voltageequals V_(dc). By controlling the turn-on of triac 370, it is possibleto control the average current supplied to the load capacitance andminimize recovery time. As V_(dc) increases with capacitor charging, aautomatically changes to keep the line current limited and undercontrol. Once the current drawn by load has returned to within allowablelimits, the relay 355 can be closed again, allowing normal operation toresume.

This approach allows us to match the allowed inrush current to the loadcharacteristic, as represented by I_(max), and the average current drawnby the load, without requiring the use of gate turn-off devices 225. Inaddition, the use of triacs 370 simplifies the gating and controlrequirements, reducing cost and complexity. Furthermore, as the triac370 and the resistors 360 and 365 are normally deactivated by relay 355and only operate during transients, the power dissipation requirementsare minimal, allowing packaging in a more compact form. Othercombinations of resistors and switching elements, such as but notlimited to triacs, can be used to control current flow.

This embodiment can also provide a soft start process for equipmentwithout built-in startup protection. Upon power-up, a two-stage softstart process is initiated. First, resistors 360 and 365 provide a highresistance to limit inrush current. After sensed current 210 subsides toan allowable level or a preset time, triac 370 is turned on to allowhigher current levels. Finally, once the current level again subsides orsufficient time has elapsed, the relay 355 is closed allowing normalload operation to begin.

FIG. 4 is an alternative embodiment of the active current surge limiterutilizing a voltage detector and an electromechanical relay. In thisnon-limiting embodiment, a normally open relay 455 is used to activatethe current limiter 140, which includes a resistor or NegativeTemperature Coefficient (NTC) thermistor 435. The NTC thermistor 435 hasa high resistance value when cold. The resistance drops dramatically asthe NTC thermistor 435 heat up, often by a factor of 10 or more,allowing higher currents to flow. The high resistance returns as the NTCthermistor 435 cools off. Manufacturers typically specify cooling timesof up to 60 seconds or more.

At startup, the relay 455 is maintained off (open) and the NTCthermistor 435 limits the inrush current that flows. As current flows,the resistance of the NTC thermistor 435 decreases providing lesscurrent limitation. After a preset time delay, the relay 455 is turnedon to de-energize the current limiter 140 by bypassing the NTCthermistor 435. This allows the NTC thermistor 435 to cool down andrestore the high resistance mode.

A detector circuit 400 is implemented that identifies when a voltage sagoccurs, and send a control signal 415 to, activate the current limiter140. One of many possible implementations of the detector circuit 400utilizes a microprocessor with an A/D converter to sense and measure theline voltage 405. The microprocessor identifies when the voltage fallsoutside a nominally acceptable boundary defined by a preset limit. Whena disturbance is detected, the detector circuit 400 sends a controlsignal 415 to a timer circuit 480, which causes the relay 455 to closeand activate the current limiter 140. As described above, the resistanceof the NTC thermistor 435 limits the surge current until the voltage isseen to return to normal conditions. After this, the NTC thermistor 435can be bypassed after a preset time. At that point, the timer circuit480 de-energizes the relay 455 bypassing the NTC thermistor 435.Incorporation of a voltage clamping device 450 provides additionalvoltage surge protection to both the connected load 120 and the activecurrent surge limiter 100.

FIG. 5 is an alternative embodiment of the active current surge limiterutilizing an optocoupler and an electromechanical relay. Thisnon-limiting embodiment uses a circuit for simulating the operation of aDC power supply in the disturbance sensor. The diode bridge 501 and thecapacitor 502 represent a typical rectifier/capacitor circuit that maybe used in a load 120. The inductance 503 and resistance 504 simulateeffective line impedance. The time constant of the load resistor 506 andcapacitor 502 is chosen to be similar to that found inrectifier/capacitor circuits. This circuit simulates the operation of ahigh power rectifier/capacitor circuit at low cost. The capacitor 502 ischarged from the line at the peaks of the sensed line voltage 505, asthe simulated load would. An optocoupler 507 is used to detect thecharging current pulse at the line voltage peaks and send a controlsignal 515 to the activator 160.

A retriggerable monostable multi-vibrator 590 with an output pulsegreater than one half cycle (8.33 mS) is triggered by the control signal515 from the optocoupler 507. As long as the charging current pulsesoccur every half cycle, the monostable multi-vibrator 590 remainstriggered. The output of the monostable multi-vibrator 590 is used toclose the relay 555 through a semiconductor switch 595, such as but notlimited to, a transistor. While the line voltage is within specifiedlimits, the relay 555 is maintained closed, de-energizing the currentlimiter 140 by bypassing a current limiting device 535, such as but notlimited to, an NTC thermistor, triac, and resistor. It should be clearto one skilled in the art that the timing and control functions could beperformed by a microprocessor or microcontroller. This implementationallows for current surge limiting without a current sensor.

If the sensed voltage 505 decreases in amplitude below the simulated DCbus voltage, the charging current pulses stop, causing the optocoupler507 to stop sending triggering pulses as the control signal 515. Whenthe triggering pulses stop, the monostable multi-vibrator 590 outputchanges state at the end of the timing period, causing switch 595 toturn the relay off after a selectable delay. This then reinserts thecurrent limiting device 535 into the circuit. When the voltage returnsto normal, the current limiting device 535 limits the inrush current tothe load 120. When the AC line voltage returns to normal, the chargingcurrent pulses begin again and the monostable multi-vibrator 590 isretriggered once again. After waiting for a preset time, the relay 555is closed once again, de-energizing or bypassing the current limiter140.

FIG. 6 is a flow chart illustrating an embodiment of a fast detectionalgorithm 600 for the active current surge limiter. Fast detectionalgorithms 600 can be implemented in, but not limited to, software,hardware and/or individual components, as illustrated in the previousembodiments of FIGS. 2-5. In this non-limiting embodiment of a fastdetection algorithm 600, the active current surge limiter 100 isenergized (610) upon starting the connected load 120. The active currentsurge limiter 100 begins sensing the power supply conditions (620). Thiscan include, but is not limited to, voltage, current, and combinationsthereof. The sensed conditions are then evaluated to determine if adisturbance exists (630). If it is determined that no disturbanceexists, then the active current surge limiter 100 continues to sense(620) and evaluate (630) the power supply condition. If a disturbancedoes exist, then the current limiter 140 is activated (640).

Once the current limiter 140 is activated, the active current surgelimiter 100 returns sensing the power supply conditions (650). Thesensed conditions are then evaluated to determine if the disturbance iscomplete (660). If it is determined that the disturbance still exists,then the active current surge limiter 100 continues to sense (650) andevaluate (660) the power supply condition. If the disturbance no longerexists, then the current limiter 140 is deactivated (670). The processrepeats until the active current surge limiter 100 and its load 120 arede-energized. Appropriate time delays, as discussed above, can beincorporated to optimize system operation and protection.

It should be emphasized that the above-described embodiments of thepresent disclosure are merely possible examples of implementations, andare merely set forth for a clear understanding of the principles of thedisclosure. Many variations and modifications may be made to theabove-described embodiments for use in single or multi-phase systems.For example, a plurality of devices can be included in the currentlimiter to provide active of passive current limitation. In addition, aplurality of circuits utilizing integrated circuits or discretecomponents can be implemented to provide disturbance sensing andactivation of the current limiter. Moreover, other automated methods todetermine voltage and current limitations can be incorporated intoactive current surge limiters. All such modifications and variations areintended to be included herein within the scope of this disclosure andprotected by the following claims.

Therefore, at least the following is claimed:
 1. A system comprising: a current limiter, including an interface configured to be connected between a power supply and a load; a disturbance sensor, configured to monitor the power supply for a disturbance during operation of the load; and an activator, configured to receive a control signal from the disturbance sensor and to activate the current limiter based on the control signal.
 2. The system in claim 1, wherein the power supply is an alternating current (AC) power supply.
 3. The system in claim 1, wherein a circuit breaker is tripped to de-energize the load when the disturbance continues beyond a preset period.
 4. The system in claim 1, wherein the current limiter includes at least one of the group consisting of: a resistor, an NTC thermistor, and a varistor.
 5. The system in claim 1, wherein the disturbance sensor monitors at least one of the group consisting of: voltage, current, and combinations thereof.
 6. The system in claim 1, wherein the disturbance is a voltage sag.
 7. The system in claim 1, wherein the disturbance is a current surge following the detection of a voltage sag.
 8. The system in claim 7, wherein a current limit is preset.
 9. The system in claim 7, wherein a current limit is determined by the disturbance sensor.
 10. The system in claim 9, wherein the current limit is based on starting current of the load.
 11. The system in claim 1, wherein the activator includes a bypass circuit.
 12. The system in claim 11, wherein the bypass circuit includes at least one of the group consisting of: an electromechanical relay, a semiconductor switch, a triac, a thyristor.
 13. The system of claim 1, wherein the current limiter is activated while the disturbance detected.
 14. A system comprising: means for limiting current supplied to a load from a power supply; means for sensing a disturbance on the power supply during operation of the load; and means for activating the means for limiting current to the load when a disturbance is sensed.
 15. The system in claim 14, wherein the system further includes means for de-energizing the load when the disturbance exceeds a preset period.
 16. A method, comprising: monitoring a condition of a power supply during operation of a load connected to the power supply; determining if the condition falls outside of an acceptable limit; and activating a current limiting device when the monitored condition falls outside of acceptable limits.
 17. The method of claim 16, wherein the monitored condition includes voltage, current, and combinations thereof.
 18. The method of claim 16, wherein the acceptable limit is determined based on variations of the monitored condition during operation of the load.
 19. The method of claim 16, wherein the current limiting device is deactivated when the monitored condition falls within acceptable limits.
 20. The method of claim 16, wherein the current limiting device is deactivated after a preset period of time. 